1. Field of the Invention
This invention generally relates to solution based deposition processes and, more particularly, to the formation of metal selenide semiconductor films using selenium nanoparticle solutions.
2. Description of the Related Art
In general, metal and mixed-metal selenides represent important classes of semiconductor materials for electronic and photovoltaic (PV) applications. In particular, copper indium gallium diselenide (CuIn1-xGaxSe2 or CIGS) has emerged as a promising alternative to existing thin-film technologies. Overall, CIGS thin films possess a direct and tunable energy band gap, high optical absorption coefficients in the visible to near-infrared (IR) spectrum and have demonstrated power conversion efficiencies (PCEs) approaching 20%. Conventional CIGS fabrication (vacuum) processes typically involve either sequential or co-evaporation (or sputtering) of copper (Cu), indium (In), and gallium (Ga) metal onto a substrate followed by annealing in an atmosphere containing a selenium vapor source to provide the final CIGS absorber layer. Alternatively, evaporation (or sputtering) of copper, indium, gallium and selenium (Se) onto heated substrates may be performed.
In contrast to vacuum approaches, which create an environment to control variables such as the gases introduced and pressure, non-vacuum methods offer significant advantages in terms of both reduced cost and high throughput manufacturing capability via roll-to-roll processing. Electrodeposition or plating of metals (from metal ions in solution) onto conductive substrates represents an alternative CIGS fabrication strategy. Finally, CIGS fabrication via deposition of mixed binary, ternary, and/or quaternary nanoparticles of copper, indium, gallium, and selenium (so called nanoparticle “inks”) embodies another non-vacuum approach.
In addition to the approaches described above, a number of alternative approaches and hybrid strategies have been reported with varying degrees of success. Overall, CIGS fabrication via solution-based approaches appears to offer a convenient, low-cost option. According to this method, metal salt (precursors) of copper, indium, and gallium are dissolved in a solvent to form a CIGS ink and subsequently deposited on a substrate to form a film using conventional methods.
Mitzi et al. described a solution-based CIGS deposition strategy using homogenous solutions of Cu, In, Ga and Se (and optionally sulfur) obtained by dissolution in hydrazine.1 Subsequently, a hydrazine-free approach was reported whereby isolated hydrazinium-based precursors could be deposited to form metal chalcogenide composite films.2 Unfortunately, the high toxicity and reactivity associated with hydrazine is a major disadvantage for these approaches. Keszler et al. described a solution-based approach for the synthesis of low contamination metal chalcogenides in aqueous media.3 In general, the formulation consists of aqueous metal chalcogenide precursors as a mixture of metal cation salts, formate anions and a source of chalcogenide (selenium, sulfur) in the form of thermally labile precursors including thiourea, thioformamide, selenourea, selenoformamide, etc. Overall, this method offers both environmentally favorable processing and low CIGS film contamination due to the careful selection of appropriate precursor materials. Finally, Wang et al. reported an inkjet printing method whereby the CIGS absorber layer was deposited by inkjet printing on molybedenum (Mo)-coated substrates from a solution of Cu, In, and Ga salts containing ethylene glycol and ethanolamine.4 Following selenization and subsequent CIGS device integration, an overall PCE of 5.04% was obtained using this approach. Subsequently, Wang et al. demonstrated CIGS solar cell performance exceeding 8% through careful optimization of Cu, In, and Ga precursor formulations.5 In general, the latter approaches employ a mixed metal salt precursor strategy which offers the advantages of low-cost and process flexibility.
Unfortunately, most conventional CIGS fabrication strategies require high temperature post-selenization following deposition of the copper, indium, and gallium (CIG) absorber layer. Even in those cases where selenium is integrated into the components before or during the film deposition phase (as in Se containing metal selenide nanoparticles or solution based process with Se precursor), selenium losses during high temperature processing can render the resultant CIGS film as selenium deficient. In addition, an inability for the selenium source vapor to penetrate deep into a deposited CIG film may lead to reduced grain size, poor overall absorber layer growth, and/or morphology as well as poor interfacial contacts.
One of the major disadvantages of high temperature selenization (H2Se gas or Se fluxes, for example) is the inherent high-toxicity, which can present serious hazards to humans in large-scale production environments. Furthermore, the high-temperature associated with the selenization processes imposes severe limitations on the types of substrates upon which CIGS can be deposited. In light of these facts, it would be advantageous to provide a method through which elemental selenium could be incorporated into the CIGS deposition as part of a solution processing approach. Conceivably, direct selenium incorporation into solution processing could offer several advantages including improved CIGS film quality, reduced thermal budget as well as considerable safety benefits.
Since elemental selenium cannot be practically employed in a solution-based approach as a powder (or other pristine form), soluble selenium precursors in the form of selenium nanoparticles (SeNPs) offer a viable alternative. Despite the novelty of the approach, there exist limited cases in which SeNPs have been suggested as viable options for a solution processed CIGS absorber layer. In some ways, this may be explained by the difficulties associated with synthesizing stable dispersions of SeNPs that can be practically utilized in a precursor solution for CIGS deposition. Since selenium is an important nutritional supplement, a majority of the prior art has focused on bio-compatible SeNPs, whereby the nanoparticles are effectively stabilized by large moieties such as hyperbranched polysaccharides, proteins, and/or high molecular weight polymers, etc. Unfortunately, the use of SeNPs stabilized by exceptionally large species is impractical due to extensive contamination by carbon, oxygen, and nitrogen species upon thermal decomposition, which prevents the realization of high-quality CIGS films. For the most part, various methods for SeNP synthesis reported in the literature represent attempts to form stable, colloidal dispersions by exhaustive measures which do not consider the implications for, and/or consequences of, additional (or practical) processing to afford functional materials.
Regardless of the target application, the fabrication of SeNPs reported in the prior art has been dominated by conventional “chemical” or “thermal” approaches whereby a soluble selenium “precursor” species is transformed to SeNPs upon the action of a chemical reagent under a specified set of conditions and, in almost every case, in the presence of an appropriate stabilizing agent (or ligand), which functions to both control SeNP growth and stabilize the resultant collection of SeNPs in solution. To a significantly lesser extent, the synthesis of SeNPs has been successfully demonstrated via microbiological processes. Below is provided a brief survey of chemical methodologies for SeNP fabrication as described in the prior art.
Mees et al. described the synthesis of selenium colloids in quantitative yield from selenous acid using sodium ascorbate as the reducing agent either in the presence of sodium dodecyl sulfate (SDS) surfactant at room temperature or, alternatively, in the absence of surfactant at elevated temperatures.6 Rajalakshmi et al. reported the synthesis of SeNPs via precipitation in a viscous polymer solution (polyacrylamide).7 Liu et al. described the preparation of SeNPs by a reverse microemulsion process using sodium selenosulfate as the selenium source.8 Lin et al. reported the fabrication of SeNPs through a mild chemical reduction method involving selenous acid, SDS (surfactant, ligand) and sulfur dioxide (SO2, reducing agent).9 Subsequently, Lin et al. described a facile, size selective method for synthesizing amorphous SeNPs at room temperature with selenous acid, SDS (surfactant, ligand) and sodium thiosulfate as reducing agent.10 Ingole et al. provided a method for a “green” synthesis of glucose-stabilized SeNPs from sodium selenosulfate at elevated temperatures.11 Zhang et al. described the fabrication of water-dispersible SeNPs from selenous acid using a hyperbranched polysaccharide (HBP) as stabilizer/capping agent in the presence of ascorbic acid.12 Chen et al. provided a process for the large scale preparation of trigonal selenium nanowires and nanotubes from sodium selenite and glucose without the need for additional templates or surfactants.13 Finally, Dwivedi et al. described a simple method for preparing SeNPs (40-100 nm) by reaction of sodium selenosulfate with various organic acids in the presence of polyvinyl alcohol (PVA) as stabilizer in aqueous media.14     1. D. B. Mitzi, W. Liu and M. Yuan, “Photovoltaic Device with Solution-Processed Chalcogenide Absorber Layer”, US2009/0145482.    2. D. B. Mitzi and M. W. Copel, “Hydrazine-Free Solution Deposition of Chalcogenide Films”, U.S. Pat. No. 8,134,150.    3. D. A. Keszler and B. L. Clark, “Metal Chalcogenide Aqueous Precursors and Processes to Form Metal Chalcogenide Films”, US2011/0206599.    4. W. Wang, Y-W. Su and C-H, Chang, “Inkjet Printed Chalcopyrite CuInxGa1-xSe2 Thin Film Solar Cells”, Solar Energy Materials & Solar Cells 2011, 95, 2616-2620.    5. W. Wang, S-Y. Han, S-J. Sung, D-H. Kim and C-H. Chang, “8.01% CuInGaSe2 Solar Cells Fabricated by Air-Stable Low-Cost Inks”, Physical Chemistry Chemical Physics 2012, 14, 11154-11159.    6. D. R. Mees, W. Pysto and P. J. Tarcha, “Formation of Selenium Colloids Using Sodium Ascorbate as the Reducing Agent”, Journal of Colloid and Interface Science 1995, 170, 254-260.    7. M. Rajalakshmi and A. K. Arora, “Optical Properties of Selenium Nanoparticles Dispersed in Polymer”, Solid State Communications 1999, 110, 75-80.    8. M. Z. Liu, S. Y. Zhang, Y. H. Shen and M. L. Zhang, “Selenium Nanoparticles Prepared from Reverse Microemulsion Process”, Chinese Chemical Letters 2004, 15, 1249-1252.    9. Z-H. Lin, F-C. Lin and C. R. C. Wang, “Observation in the Growth of Selenium Nanoparticles”, Journal of the Chinese Chemical Society 2004, 51, 239-242.    10. Z-H. Lin and C. R. C. Wang, “Evidence on the Size-Dependent Spectral Evolution of Selenium Nanoparticles”, Materials Chemistry and Physics 2005, 92, 591-594.    11. A. R. Ingole, S. R. Thakare, N. T. Khati, A. V. Wankhade and D. K. Burghate, “Green Synthesis of Selenium Nanoparticles Under Ambient Conditions”, Chalcogenide Letters 2010, 7, 485-489.    12. Y. Zhang, J. Wang and L. Zhang, “Creation of Highly Stable Selenium Nanoparticles Capped with Hyperbranched Polysaccharide in Water”, Langmuir 2010, 26, 17617-17623.    13. H. Chen, D-W. Shin, J-G. Nam, K-W. Kwon and J-B. Yoo, “Selenium Nanowires and Nanotubes Synthesized via a Facile Template-Free Solution Method”, Materials Research Bulletin 2010, 45, 699-704.    14. C. Dwivedi, C. P. Shah, K. Singh, M. Kumar and P. N. Bajaj, “An Organic Acid-Induced Synthesis and Characterization of Selenium Nanoparticles”, Journal of Nanotechnology 2011, Article ID 651971.
It would be advantageous if a method existed through which selenium, in the form of nanoparticles, could be incorporated into a solution processing approach for the fabrication of metal selenide containing semiconductor materials including CIGS.